Zimbabwe's parliament began debating a change to the constitution yesterday that will permit President Robert Mugabe to appoint his successor without holding a general election.

The opposition had criticised the proposal, which it said was intended to ensure that the ruling Zanu-PF retains power for years to come despite its deepening unpopularity. But yesterday the Movement for Democratic Change said it would not oppose the measure because the government agreed to abolish the president's power to appoint unelected members to the lower house of parliament, and because the change could lead to wider constitutional reform that will eventually ease Mr Mugabe from power.

Thokozani Khupe, deputy leader of one of the two rival MDC factions in parliament, said the party also wanted to show support for talks mediated by South Africa's president, Thabo Mbeki.

"As a confidence-building measure, we do not stand in the way of the constitutional amendment," she said. "But we are in no way abandoning our principle for a new people-driven constitution."

The move comes as a report by the International Crisis Group (ICG) said that Zimbabwe was "closer than ever to complete collapse" and threatened to destabilise the rest of southern Africa.

"Four out of five of the country's 12 million people live below the poverty line and a quarter have fled, mainly to neighbouring countries," the thinktank said. "Through repression and patronage, Mugabe can still control politics but not the deteriorating economy - including runaway inflation - which is hurting the region more than ever before."

Inflation is officially recorded at about 6,600%, but with most trade now on the black market, economists believe the real figure to be several times higher.

The ICG said Zimbabwe's neighbours in the Southern African Development Community should do more to pressure Mr Mugabe to step down but that it might take a guarantee of immunity from prosecution for him and other Zanu-PF leaders, and permission to keep their wealth, to force the president from office.

"The incentives that may be needed to achieve these outcomes are likely to be controversial, though not necessarily among Zimbabweans, who want above all an end to their nightmare," the report said. "In exchange, Mugabe would have to retire at the end of his term (March 2008), and the security establishment would have to accept political reforms and free and fair elections."

But the constitutional changes suggest that Mr Mugabe, 83, is intent on ensuring that if he leaves the office he has held since 1980 his party remains in power, which would provide the best guarantee against being called to account for abuses.

The change would combine presidential and parliamentary elections in March 2008 and then allow Mr Mugabe to impose his successor, without a further vote, if he should retire before his subsequent five-year term as president is complete.

Parliament is also expected to redraw the country's constituency boundaries to favour Zanu-PF.

"The increase in parliamentary seats is not justified by demographic growth," said the ICG. "It is a calculated strategy to reconfigure parliament to make it more amenable and guarantee the status quo will last beyond Mugabe's own rule."

The ICG also criticised intervention by Britain and other western powers, saying that sanctions against Zimbabwe's leaders had proved ineffective. "General condemnations from the UK and US if anything [are] counterproductive because they help Mugabe claim he is the victim of neocolonial ambitions," it said.

The group called on foreign governments to throw their weight behind Mr Mbeki's mediation efforts.